The invention relates to Niobium (Nb)-silicide based composite compositions. In particular, the invention relates to Nb-silicide based composite compositions having properties that permit the Nb-silicide based composite compositions to find applications in turbine components.
Turbines and their components (hereinafter “turbine components”), such as, but not limited to, aeronautical turbines, land-based, turbines, marine-based turbines, and the like, have typically been formed from nickel (Ni)-based materials, which are often referred to as Ni-based superalloys. Turbine components formed from these Ni-based superalloys exhibit desirable chemical and physical properties under the high temperature, high stress, and high-pressure conditions generally encountered during turbine operation. For example, turbine components, such as an airfoil, in modern jet engines can reach temperatures as high as about 1,150° C., which is about 85% of the melting temperatures (Tm) of most Ni-based superalloys.
Because Ni-based superalloys have provided the level of performance desired in such applications, the development of such Ni-based superalloys has been widely explored. Consequently, the field has matured and few significant improvements have been realized in this area in recent years. In the meantime, efforts have been made to develop alternative turbine component materials. These alternate materials include niobium (Nb)-based refractory metal intermetallic composites (hereinafter “RMIC”s). Most RMICs have melting temperatures of about 1700° C. If RMICs can be used at about 80% of their melting temperatures, they will have potential use in applications in which the temperature exceeds the current service limit of Ni-based superalloys.
RMICs comprising at least niobium (Nb), silicon (Si), titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al) have been proposed for turbine component applications. These silicide-based RMICs exhibit a high temperature capability that exceeds that of current Ni-based superalloys. Exemplary silicide-based RMICs are set forth in U.S. Pat. No. 5,932,033, to M. R. Jackson and B. P. Bewlay, entitled “Silicide Composite with Nb-Based Metallic Phase and Si-Modified Laves-Type Phase” and U.S. Pat. No. 5,942,055, to Jackson and Bewlay, entitled “Silicide Composite with Nb-Based Metallic Phase and Si-Modified Laves-Type Phase”.
Some known Nb-silicide based composites—including silicide-based RMIC materials—have adequate oxidation resistance characteristics for turbine component applications. These materials have compositions within the following approximate ranges: 20-25 atomic percent titanium (Ti), 1-5 atomic percent hafnium (Hf), and 0-2 atomic percent tantalum (Ta), where the concentration ratio (Nb+Ta):(Ti+Hf) has a value of about 1.4; 12-21 atomic percent silicon (Si), 2-6 atomic percent germanium (Ge), and 2-5 atomic percent boron (B), where the sum of the Si, B, and Ge concentrations is in the range between 22 atomic percent and 25 atomic percent; 12-14 atomic percent chromium (Cr) and 0-4 atomic percent iron (Fe), where the sum of the Fe and Cr concentrations is between 12 atomic percent and 18 atomic percent; 0-4 atomic percent aluminum (Al); 0-3 atomic percent tin (Sn); and 0-3 atomic percent tungsten (W). Other known Nb-based silicide composites—including silicide-based RMIC materials—possess adequate creep-rupture resistance for turbine component applications. These materials have compositions within the following approximate ranges: 16-20 atomic percent Ti, 1-5 atomic percent Hf, and 0-7 atomic percent Ta, where the concentration ratio (Nb+Ta):(Ti+Hf) has a value of about 2.25; 17-19 atomic percent Si, 0-6 atomic percent Ge, and 0-5 atomic percent B, where the sum of the Si, B, and Ge concentrations is in the range between 17 atomic percent and 21 atomic percent; 6-10 atomic percent Cr and 0-4 atomic percent Fe, where the sum of the Fe and Cr concentrations is in the range between 6 atomic percent and 12 atomic percent; 0-4 atomic percent Al; 0-3 atomic percent Sn; 0-3 atomic percent W; and 0-3 atomic percent Mo. In addition, some known Nb-silicide based composites—including silicide-based RMIC materials—have adequate fracture toughness for turbine component applications. Such materials contain greater than or equal to about 30 volume percent of metallic phases present in such components.
Although the above Nb-silicide based composites and silicide-based RMIC materials possess beneficial mechanical and chemical properties, they do not adequately balance oxidation resistance properties with toughness and creep resistance properties. Thus, a single silicide-based RMIC alloy material composition that can provide adequate creep, oxidation resistance, and toughness for turbine component applications is currently not available.
While the oxidation performance and creep-rupture resistance for turbine component applications of known RMICs are desirable, these materials and their properties may still be further improved for turbine component applications. For example, the chemistries and compositions of the RMIC material may be modified to enhance oxidation resistance for applications that subject the turbine component to high stresses at temperatures ranging from about 1300° F. to about 1700° F. (about 700° C. to about 925° C.) over extended periods of time.
Therefore, what is needed is a material having a composition, chemistry, and properties that are suitable for various applications such as, but not limited to, turbine components, in which high stresses at elevated temperatures are encountered over long periods of time. More particularly, what is needed is a Nb-silicide based RMIC having improved oxidation resistance and creep resistance for use in turbine component applications in which high stresses at elevated temperatures are encountered over long periods of time.